Silica sols are made by adding sodium or potassium silicate to a dilute aqueous dispersion of small silica particles or nuclei and removing the sodium or potassium ions from the dispersion. This results in silicic acid being deposited upon the nuclei which grow to larger size and thus increase the silica concentration of the sol. In all processes of the prior art for making concentrated sols of silica particles of about 10 nm or less in diameter, it has been necessary to grow the particles in a dilute solution and then concentrate them by the evaporation of water which requires energy. In the following prior art the final sols contain particles larger than 10 nm in diameter.
There are at least three quite feasible processes by which silica sols can be made economically. These can be classified by the method of removing the cations from the dilute aqueous dispersion: (1) with ion exchange resins; (2) nonelectrolytically through membranes; and (3) by electrodialysis.
The use of ion exchange resins for making silica sols is well known and is described in U.S. Pat. Nos. 2,244,325 to Bird and U.S. Pat. No. 2,631,134 to Iler et al. U.S. Pat. No. 3,789,009 to Irani describes the use of ion exchange resins at high temperatures (60.degree.-150.degree. C.) to obtain dilute (10-16% SiO.sub.2) "large particle" silica sols having particle number average diameter greater than about 15 nm which can be concentrated by evaporation. U.S. Pat. No. 2,601,235 to Alexander describes the processes in which nuclei of high molecular weight silica preferably made by heating a heel of silica sol above 60.degree. C., are mixed with active silica and heated above 60.degree. C. to obtain dilute built-up silica sols. Within the temperature range disclosed in the art (60.degree.-150.degree. C.) the initial number of silica nuclei decreases and the larger particles increase in average size at the expense of the smaller particles which dissolve at high temperatures. Thus, the number of nuclei during the heat-up period is not maintained constant in number and the size of the nuclei is not kept small so that the final particles are larger than 10 nm (10 millimicrons) in diameter, e.g., 15 to 130 nm (Column 6, line 1).
Non electrolytic removal of the cations through membranes is described in U.S. Pat. No. 3,756,958 to Iler.
U.S. Pat. No. 3,668,088 to Iler describes the preparation by electrodialysis at high temperature of aqueous sodium or potassium silicate, of dilute silica sols of large particle size and very dilute sols of small particle size larger than 10 nm which can be concentrated by evaporation. The patent recites that at lower temperatures only extremely small particles of colloidal silica are obtained and the sols can therefore not be concentrated without gelling (Column 2, lines 27-29). Again in this case, the number of nuclei was not kept constant during the heat-up period so that in all examples the final particle size is larger than 10 nm.
The above-referred growth of silica particles by further accretion of silica as taught by the prior art is carried out in "hot solution", i.e., 50.degree.-100.degree. C., for a variety of reasons:
(1) In the electrodialysis process, to reduce the electrical resistance of the electrolyte and thus reduce the power required; PA1 (2) In the ion exchange process when using a weak acid type of cation-exchange resin, to accelerate the rate of removal of sodium from solution; and PA1 (3) To increase the rate at which soluble or active silica, released from soluble alkali metal silicate by removal of alkali metal ion, is deposited upon silica particles present in the solution. PA1 (a) forming an aqueous solution of alkali metal silicate containing 0.2-3.0% weight alkali metal silicate, basis SiO.sub.2 in water, in which the molar ratio of SiO.sub.2 to alkali metal oxide is from 2.5:1-3.9:1; PA1 (b) at a temperature of 10.degree.-50.degree. C., reducing the solution pH to 8-10.5, thereby spontaneously forming silica nuclei particles which are dispersed in the solution; and PA1 (c) removing alkali metal ions from the nuclei-containing alkali metal silicate solution while (1) maintaining the solution at pH 8-10.5 by addition of alkali metal silicate at a rate sufficient to maintain a constant number of particles and (2) simultaneously raising the temperature of the solution by at least 10.degree. C. to 50.degree.-100.degree. C. PA1 (1) the more rapidly the heating is carried out, the smaller the particles will be when the growth temperature is reached; and PA1 (2) the more rapidly the heating is carried out, the faster must be the rate of silicate addition.
Basically, nuclei particles for the above-described methods of making silica sols are formed whenever a dilute aqueous solution of soluble silicate, which has a pH of over 11, is reduced in pH to below about 10.5. The lower the temperature, the smaller are the resultant nuclei.
Heretofore, it has been the practice to take such nuclei dispersions, heat them to "hot solution" temperature and then to enlarge the particles by silica accretion as described above.
However, when nuclei made at a lower temperature are heated to the higher temperatures (50.degree.-100.degree. C.) at which particle growth is to be carried out, the nuclei are not stable especially when the particle diameter is less than about 5 nm. Smaller particles are dissolved and the remaining particles increase in average size.
The art processes effect the release of active silica under conditions of pH, temperature and rates of addition of active silica which do not result in the formation of additional silica nuclei, but in a decrease of the total number of silica nuclei. Therefore, the total number of nuclei present in the system is not kept constant during the process. When soluble silicate is added to the sol to grow the particles and to increase silica concentration, the final particle size depends on the number of surviving nuclei.
If too few nuclei survive the heating, the added active silica will be deposited in a smaller number of particles and therefore there will be faster growth and the final particles may be too large for certain applications, e.g., as a binder for ceramic bodies and refractory fibrous insulation. For those reasons, there has been a need for a way to make dispersions of nuclei particles of about 10 nm or less in diameter and then to prevent their being dissolved at the higher temperature needed for efficient particle growth. This has proved to be a serious problem especially when concentrated sols of very small particle size (e.g., 15 or 20% SiO.sub.2, particles smaller than about 10 nm in diameter) are to be made by electrodialysis. However, it is also difficult to produce similar concentrated sols of small particles directly by ion exchange. Dilute sols of small particles can be made by these processes and such sols can be concentrated by evaporation of water, but this requires the expenditure of extra energy. Uniformity of particle size is important in the case of product sols of particles smaller than about 6 or 7 nm in diameter in order to avoid spontaneous growth in size during storage. If there is a spread in size, the particles increase in average size but decrease in number, thereby changing in properties.
This invention teaches a procedure to maintain constant the original number of nuclei particle in the sol while the particles are grown so that no nuclei are lost in the growth process and therefore more concentrated products of small and quite uniformly sized silica particles are obtained.